Thanks for the template file containing the chart.
Please see the sample code below for your reference. I have used the existing data source in your template file for the chart, I have added a new sheet to the workbook and inserted the stacked column chart to whose horizontal axis has multiple level labels as per your requirements. Please refer to the sample code and write/ update your own codes accordingly for your needs.
Workbook workbook1 = new Workbook(“e:\test2\TestChart.xlsx”);
Worksheet sheet = workbook1.Worksheets[workbook1.Worksheets.Add()];
Cells cells = sheet.Cells;
int chartIndex = sheet.Charts.Add(ChartType.ColumnStacked, 1, 1, 25, 15);
Chart chart = sheet.Charts[chartIndex];
chart.NSeries.CategoryData = “Sheet1!$B$31:$D$42”;
for (int i = 0; i < chart.NSeries.Count; i++)
chart.NSeries[i].GapWidth = 150;
chart.NSeries[i].Overlap = 100;
chart.CategoryAxis.HasMultiLevelLabels = true;
chart.Title.Text = “Test Chart1”;
Hope, this helps a bit.